Light emitting device driving circuit

ABSTRACT

The circuit comprises an amplitude setting transistor  5 Q for controlling the amplitude of high-frequency current I 2 QB flowing through the second current mirror circuit  2 , by using an input of reference direct current signal Bias. Direct current component I 4 QB generated based on reference direct current signal Bias is subtracted from direct current I 1  flowing through the other side line of the first current mirror circuit  1 . In this case, level fluctuation of driving current IZ can be significantly suppressed, since the increment of the direct current component included in high-frequency current I 2 QB is proportional to direct current component I 4 QB which is subtracted from direct current I 1  flowing through said other side line of the first current mirror circuit  1.

TECHNICAL FIELD

The present invention relates to light emitting device driving circuits.

BACKGROUND ART

An Optical pickup is a device which can read information stored onstorage media such as CDs or a DVDs by irradiating laser light fromlaser diodes onto the storage media and monitoring the light reflectedtherefrom. Laser diodes may also be used for writing information onstorage media. Heretofore, driving circuits for such laser diodes havebeen developed.

DISCLOSURE OF THE INVENTION

In an optical pickup, unnecessary light or background light reflected inthe apparatus may enter the photo-detector to become noise contained inrequired information. In addition, when the light reflected in storagemedia returns to a light emitting device and enters the light emittingdevice, light emission of the light emitting device may become unstable,causing a noise.

Accordingly, it is conceivable that an approach for driving lightemitting device at high-frequency causes noise tolerance to improve.High-frequency current is superimposed onto direct current componentsand supplied to light emitting device as driving current. The preferredfrequency of high-frequency current applicable to present CD players isof 300-500 MHz.

It is also conceivable not to provide such high-frequency current to thelight emitting device to control power consumption. In addition, themean value of high-frequency current in itself may also vary dependingon temperature and aged deterioration. Thus, as the mean value of thehigh-frequency current varies, the level of the driving current changes.Since a slight level fluctuation may increase the probability ofdetection errors, smaller level fluctuation is preferred.

The present invention is achieved in consideration of theabove-mentioned problems, and therefore an object of the presentinvention is to provide a light emitting device driving circuit capableof suppressing level fluctuation of the driving current in a state,where the light emitting device driving circuit superimposeshigh-frequency current components onto direct current components togenerate a driving current which in turn is supplied to the lightemitting device.

In order to address the above-mentioned problems, a light emittingdevice driving circuit according to the present invention comprising afirst and a second current mirror circuits each of which having a pairof parallel lines in a state, where each one side line of said parallellines is connected to said light emitting device; applying directcurrent and high-frequency current respectively to the other side linesof the line pairs in the first and the second current mirror circuits;and supplying, through a node of the connection, driving currentgenerated by superimposing high-frequency current onto direct current,wherein, there is provided an amplitude setting transistor forcontrolling, by using an input of reference direct current signal, theamplitude of the high-frequency current flowing through the secondcurrent mirror circuit so that direct current components generated basedon the reference direct current signal may be subtracted from the directcurrent flowing through said other side line of said line pair of thefirst current mirror circuit.

More specifically, as the amplitude of the high-frequency current in thesecond current mirror circuit increases, the direct current component ofthe high-frequency current increases, thereby increasing the mean valueof the driving current given as the sum of the currents respectivelyflowing through each one side line of the first and the second currentmirror circuits.

When reference direct current signal which is input into the amplitudesetting transistor increases, the amplitude of the high-frequencycurrent flowing through the second current mirror circuit increasesthereby increasing the mean value of the driving current, whilefluctuation of the mean value of the driving current may be controlledbecause the direct current component generated from the reference directcurrent signal is subtracted from the direct current flowing through theother side line of the first current mirror circuit. That is to say,since the driving current is generated by superimposing the directcurrent which is equivalent or proportional to the direct currentflowing through the other side line of the first current mirror circuitonto the high-frequency current, the above-mentioned direct currentcomponent is subtracted from the driving current, whereby fluctuation ofthe mean value of the driving current can be suppressed.

In addition, level fluctuation of the driving current can besignificantly suppressed when the increment of the direct currentcomponent of said high-frequency current is set to be proportional tothe direct current component which will be subtracted from the directcurrent flowing through said other side line of said first currentmirror circuit.

A light emitting device driving circuit of the present inventioncomprises a variable current source for providing current to a node onsaid side line in said first current mirror circuit, and preferablycomprises a constant current source for keeping the amount of currentsupplied from this side line and said variable current source at aconstant value.

In this case, depending on the amount of the current supplied from thevariable current source, the current flowing through one and the otherside lines of the first current mirror circuit, i.e., the size of thedirect current component supplied to the light emitting device, can becontrolled.

Furthermore, in the light emitting device driving circuit of the presentinvention, said variable current source is composed of a current mirrorcircuit, with a transistor disposed downstream of the input side line ofthe current mirror circuit, wherein upon application of a currentcontrol voltage to the control terminal of the transistor, the amount ofthe current supplied from the variable current source, i.e., the size ofthe direct current component supplied to the light emitting device canbe controlled, depending on the level of the current control voltage.

In addition, the light emitting device driving circuit of the presentinvention comprises a high-frequency generation circuit for generatingsaid high-frequency current, said amplitude setting transistorpreferably being connected to said high-frequency generation circuit sothat the upstream electric potential thereof determines the amplitude ofsaid high-frequency current.

In this case, the amplitude of the high-frequency current componentsupplied to the light emitting device can be controlled by adjusting theupstream electric potential of the amplitude setting transistor.

In the light emitting device driving circuit of the present invention, acommon-mode voltage is preferably input into the control terminal ofsaid transistor disposed downstream of the input side line of saidvariable current source, and the control terminal of said amplitudesetting transistor.

In this case, direct current and high-frequency current supplied to thelight emitting device can be altered in phase, since the input voltageto the control terminal of said transistor and the input voltage to thecontrol terminal of said amplitude setting transistor have a commonmode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of the light emitting device drivingcircuit.

FIG. 2 is a graph of current I2QA flowing in one of the side lines ofthe high-frequency generation circuit 3.

FIG. 3 is a graph illustrating the wave form of driving current IZ.

BEST MODES FOR CARRYING OUT THE INVENTION

The light emitting device driving circuit according to an embodiment isdescribed below. Like elements will be designated by like numerals andredundant description will be omitted.

FIG. 1 is a circuit diagram of a light emitting device driving circuit.The current mirror circuit, composed by connecting the control inputterminals of two transistors, has two parallel lines through whichcurrents of the transistors flow respectively.

A plurality of current mirror circuits 1, 2 and 4 are described in thefollowing description, wherein current mirror circuits 1, 2 and 4respectively have, as shown in the case of FET, transistor pairs 1QA,1QB; 2QA, 2QB; 4QA, 4QB, each gate thereof being connected as thecontrol input terminal. Here, in the case of a bipolar transistor, thecontrol input terminal functions as the base.

The present driving circuit comprises a first current mirror circuit 1(circuit for supplying direct current) and a second current mirrorcircuit 2 (circuit for supplying high-frequency current), each having apair of parallel lines. Each one side line in current mirror circuits 1and 2 is connected to a light emitting device(load) Z.

By passing direct current and high-frequency current respectivelythrough the other side lines of said parallel lines in the first and thesecond current mirror circuits 1 and 2, the present driving circuitsupplies driving current IZ generated by superimposing direct currentonto high-frequency current to light emitting device Z, through aconnection node Y.

First the configuration of the part of direct current will be described.A direct current source I1 (from which current I1 may flow) is connectedat a downstream region of the other side line in the first currentmirror circuit 1, so that direct current component I4QB flows betweenthis side line and the current source I1 through node X. Accordingly,current I1QA, generated as a result of subtracting direct currentcomponent I4QB from direct current I1, flows through an upstream regionbeyond node X of the side line, whereas current I1QB which is equivalentor proportional to current I1QA flows through the counterpart side line.

In essence, direct current I1QB supplied to light emitting device Z isgenerated as a result of subtracting a predetermined direct current I4QBfrom direct current I1. The determined direct current component I4QB isgenerated in current mirror circuit 4 for generating a direct currentcomponent. In other words, direct current component I4QA which isequivalent or proportional to direct current component I4QB flowsthrough transistor 4QA which is the counterpart of transistor 4QBthrough which direct current component I4QB flows. Direct currentcomponent I4QA is proportional to a reference direct current signal(control voltage) Bias which is input to the control input terminal(gate) of direct current setting transistor 6Q disposed downstream oftransistor 4QA.

Accordingly, direct current component I4QA (I4QB) generated based onreference current signal Bias is subtracted from direct current I1flowing through the other of said side lines in the first current mirrorcircuit 1.

The configuration of the high-frequency side will be described next.Reference direct current signal Bias is also input into the controlinput terminal (gate) of amplitude setting transistor 5Q, therebypassing direct current I5Q through amplitude setting transistor 5Q.Direct current I5Q flowing through amplitude setting transistor 5Q isproportional to the amplitude of high-frequency current generated inhigh-frequency generation circuit (differential current switch) 3.

In other words, high-frequency generation circuit 3 has an oscillatingcircuit OSC for mediation between the control input terminals (gates) ofa pair of transistors 3QA and 3QB, with an amplitude setting transistor5Q being connected to the downstream side of transistors 3QA and 3QB todetermine the aggregate sum (amplitude, direct current component) ofcurrents I3QA and I2QA flowing through transistors 3QA and 3QB,respectively.

FIG. 2 is a graph showing current I2QA flowing through one of the sidelines of high-frequency generation circuit 3. High-frequency generationcircuit 3 completely separates current I5Q into I3QA and I2QA accordingto the phase of an oscillating circuit OSC. Thus, I2QA becomes a pulsecurrent having a peak value corresponding to I5Q, and consequently, hasa direct current component of I5Q/2.

Current I2QA flowing through transistor 3QB, one of the two transistorsin the high-frequency generation circuit 3, is equivalent to the currentflowing through said other side line of the second current mirrorcircuit 2 for high-frequency. Thus, current I2QB which is equivalent orproportional to current I2QA flows through transistor 2QB and issupplied to light emitting device Z through node Y, transistor 2QB beingthe counterpart of transistor 2QA through which current I2QA flows.

In essence, amplitude setting transistor 5Q controls the amplitude ofhigh-frequency current I2QA(I2QB) flowing through the second currentmirror circuit 2 according to the input of reference direct currentsignal Bias.

When the amplitude of high-frequency current I2QA(I2QB) in the secondcurrent mirror circuit 2 increases, the direct current component ofwhich increases, and consequently the mean value of driving current IZgiven as the sum of the currents respectively flowing through each oneside line of both the first and the second mirror circuits 1, 2increases.

When reference direct current signal Bias which is input into amplitudesetting transistor 5Q increases, the amplitude of high-frequency currentI2QA(I2QB) flowing through the second current mirror circuit 2 increasesand the mean value of driving current IZ increases, while fluctuation ofthe mean value of driving current IZ will be suppressed since directcurrent component I4QA(I4QB) generated from reference direct currentsignal Bias is subtracted from direct current I1 flowing through theother side line of the first current mirror circuit 1.

In other words, since driving current IZ results from superimposingdirect current I1QB which is equivalent or proportional to directcurrent I1QA flowing through the other side line of the first currentmirror circuit 1 onto high-frequency current I2QB, direct currentcomponent I4QA(I4QB) is subtracted from driving current IZ, wherebyfluctuation of the mean value of driving current can be suppressed.

Here, description will be provided about the current flowing througheach transistor. Let us assume that I1QB=G1*I1QA, I2QB=G2*I2QA,I4QB=G4*I4QA, where G1, G2, and G4 are gain ratios of the transistors,with a value of 1, for example. I1QB=I1−I4QB holds, since I1QB and I4QBwill be added at node X to give I1. Here, driving current IZ is anaddition of currents I1QB and I2QB, each current being added at node Y.

Transistors 5Q and 6Q have a common gate, which is connected toreference direct current signal Bias. Each current is controlled byreference direct current signal Bias. Let coefficient G56 be the gainratio between transistors 5Q and 6Q, the size of each transistor isdetermined so that the current ratio will be I6Q=G56*I5Q. The amplitudeof high-frequency current is controlled by reference direct currentsignal Bias.

In other words,

$\begin{matrix}{{IZ} = {{I\; 1{QB}} + {I\mspace{11mu} 2{QB}}}} \\{= {{G\; 1*I\; 1{QA}} + {I\; 2{QB}}}} \\{= {{G\; 1*\left( {{I\; 1} - {I\; 4{QB}}} \right)} + {I\; 2{QB}}}} \\{= {{G\; 1*\left( {{I\; 1} - {G\; 4*I\; 4{QA}}} \right)} + {G\; 2*I\; 2{QA}}}} \\{= {{G\; 1*\left( {{I\; 1} - {G\; 4*G\; 56*I\; 5Q}} \right)} + {G\; 2*I\; 2{QA}}}} \\{= {{G\; 1*I\; 1} - {G\; 1*G\; 4*G\; 56*I\; 5Q} + {G\; 2*I\; 2{QA}\mspace{14mu}{{holds}.}}}}\end{matrix}$

Let <IZ> denote the temporal mean value of driving current IZ, then,<IZ>=G 1*I 1−G 1*G 4*G 56*I 5 Q+G 2*(I 5 Q/2)holds (equation A). Here, the temporal mean value of IZ (level) is setso as not to be varied by reference direct current signal Bias. In otherwords, it is sufficient that the second and the third terms of equationA cancel each other in order to keep the temporal mean value of drivingcurrent IZ invariant against the change of the amplitude ofhigh-frequency current.

Thus, G1*G4*G56=G2/2 holds. In this case <IZ>=G 1*I1. G2=2 holdsassuming that G1=G4=G56=1.

In other words, level fluctuation of driving current IZ can besignificantly suppressed, since the increment (caused by the increase ofamplitude) of the direct current component included in high-frequencycurrent I2QB is set to be proportional (or equivalent) to direct currentcomponent I4QB which is subtracted from direct current I1 flowingthrough the other side line of the first current mirror circuit 1.

FIG. 3 is a graph showing the waveform of driving current IZ when theabove equations hold. As can be seen from the graph, mean current <IZ>is invariant against the change of the amplitude of driving current IZ.

As described above, since fluctuation of the temporal mean value ofdriving current IZ is suppressed in the above mentioned light emittingdevice driving circuit, the intensity of laser light does not varyregardless of the presence of high-frequency superimposing when thelight emitting device driving circuit is driven with light emittingdevice Z as the laser diode, which is particularly effective when usinghologram in the optical system.

In addition, for an optical disk memory, laser light is irradiated ontothe disk surface to read information. Laser diodes are used as the lasersource, in which case reflected light from the disk returns to the LD,causing noise. The above-mentioned light emitting device driving circuitis also effective when superimposing high-frequency (300-500 MHz) as asolution since level fluctuation is suppressed.

As described above, the above-mentioned light emitting device drivingcircuit comprises a variable current source 4 for supplying current tonode X on the other side line in the first current mirror is circuit 1,and a constant current source I1 for preserving the sum of current valueI1QA flowing through this side line and current value I4QB supplied fromvariable current source 4 to a constant value.

In this case, depending on the amount of current I4QB supplied fromvariable current source 4, the amount of current flowing through one andthe other side line of the first current mirror circuit 1, i.e., thesize of direct current component supplied to light emitting device Z canbe controlled.

In addition, variable current source 4 is composed of current mirrorcircuit 4, with light emitting device driving circuit having atransistor 6Q disposed downstream of input side line of current mirrorcircuit 4, and when current control voltage Bias is applied to thecontrol terminal (gate) of transistor 6Q, the amount of current I4QBsupplied from variable current source 4, i.e., the size of directcurrent component supplied to light emitting device Z can be controlleddepending on the size of this current control voltage Bias.

In addition, the light emitting device driving circuit of the presentinvention comprises high-frequency generation circuit 3 for generatinghigh-frequency current, wherein amplitude setting transistor 5Q isconnected to high-frequency generation circuit 3 so that the upstreamelectric potential determines the amplitude of high-frequency current.

Thus, the amplitude of high-frequency current component supplied tolight emitting device Z can be controlled by adjusting upstream electricpotential of transistor 5Q using voltage Bias.

In addition, in the light emitting device driving circuit, a common modevoltage Bias is input into the control terminal of transistor 6Q whichis disposed downstream of the input side line of variable current source4 and the control terminal of amplitude setting transistor 5Q.

In this case, direct current and high-frequency current supplied tolight emitting device Z can be altered in phase, since input voltageBias to the control terminal of transistor 6Q and input voltage Bias tothe control terminal of amplitude setting transistor 5Q have a commonmode.

As described above, level fluctuation of driving current can besuppressed according to the light emitting device driving circuit of thepresent invention.

INDUSTRIAL APPLICABILITY

The present invention can be used for light emitting device drivingcircuits.

1. A light emitting device driving circuit comprising: first and secondcurrent mirror circuits each of which having a pair of parallel lines ina state, where each one side line of said parallel lines is connectedvia a connection node to said light emitting device; applying directcurrent and high-frequency current respectively to the other side linesof the line pairs in said first and second current mirror circuits, adirect current source being connected at a downstream region of theother side line of the first current mirror circuit; and supplying,through the connection node, driving current generated by superimposinghigh-frequency current onto direct current, wherein, there is providedan amplitude setting transistor for controlling, by using an input ofreference direct current signal, the amplitude of the high-frequencycurrent flowing through said second current mirror circuit so that adirect current component generated based on said reference directcurrent signal is subtracted from the direct current flowing from thedirect current source in said other side line of said line pair of saidfirst current mirror circuits, wherein the increment of the directcurrent component of said high-frequency current is set to beproportional to the direct current component subtracted from the directcurrent flowing from the direct current source in said other side lineof said line pair of said first current mirror circuit.
 2. A lightemitting device driving circuit according to claim 1, comprising avariable current source for supplying current to a node in said otherside line in said first current mirror circuit, wherein the directcurrent source is configured to preserve the current value supplied fromthis line and said variable current source to a constant value.
 3. Alight emitting device driving circuit according to claim 2, saidvariable current source being composed of a current mirror circuit, witha transistor disposed downstream of input side line of the currentmirror circuit, wherein a current control voltage is applied to thecontrol terminal of the transistor.
 4. A light emitting device drivingcircuit according to claim 3, comprising a high-frequency generationcircuit for generating said high-frequency current, wherein saidamplitude setting transistor is connected to said high-frequencygeneration circuit so that the upstream electric potential determinesthe amplitude of said high-frequency current.
 5. A light emitting devicedriving circuit according to claim 4, wherein a common mode voltage isinput to the control terminal of said transistor disposed downstream ofthe input side line of said variable current source, and the controlterminal of said amplitude setting transistor.